Phased array antennas are used for both transmitting and receiving radio frequency (RF) communications signals. Phased array antennas can transmit to conventional-antenna-based receivers, or vice versa. When a phased array antenna is used for both ends of the communication link, both the transmitter and receiver create a beam with a gain in the proper direction (pointing at each other) and then modifying that direction (steering the beam) as the transmitter and/or receiver moves or changes direction. This changing direction or moving can be the result of transmitter and/or receiver platform motion or vibration, atmospheric effects, multi-path effects or other movement. A similar situation exists when the phased array antenna is used only for transmission, except that steering of the receiver antenna may in fact be done mechanically, or the antenna may not be steered at all if the antenna is a fixed antenna. In this latter case, all beam steering would be done by the phased array antenna associated with the transmitter. A typical example would be an aircraft with a phased array antenna that is transmitting to or receiving a transmission from a satellite and must track the satellite through all of the maneuvers of the aircraft. Or it could be two helicopters communicating with each other as they both fly on their separate paths, each with its own phased array antenna.
The standard method for a receiver to track a transmitted signal to the receiver to improve the communications performance uses the signal power and tries to continuously maximize that signal power by steering the phased array receive beam or by mechanically steering the antenna. A disadvantage of current power-based receiver tracking methods is that with only one signal power measurement available at a time, any loss or gain could indicate that the beam's direction must be corrected. However, no indication is available concerning which direction the beam should be steered. Thus typical beam steering algorithms must steer off in other directions in order to sample the signal power in those directions in order to update the steering vector and hence the beam direction. Typically, these algorithms would essentially track the signal at a lower average signal power than is available with a correctly pointed beam. For example, the tracking might occur at the 3 decibel (dB) level (one half power) by tracking two, three or four points around the actual maximum power direction at the 3 dB level. This in turn produces a “pointing loss” and “gain ripple”. These ripples are on top of the normal signal power ripple caused by propagation, scintillation and multi-path effects which are not caused by beam pointing inaccuracy. The gain loss and ripples directly affect the signal quality and must be taken into consideration in any system. Accordingly, there is a need for beam tracking or steering that is not subject to these disadvantages.